When is a molecule chiral

Figure 2: Comparison of Chiral and Achiral Molecules. Rotation of its mirror image does not generate the original structure. To superimpose the mirror images, bonds must be broken and reformed. Note that even if one were to flip over the left molecule over to the right, the atomic spatial arrangement will not be equal.

This is equivalent to the left hand - right hand relationship, and is aptly referred to as 'handedness' in molecules. This can be somewhat counter-intuitive, so this article recommends the reader try the 'hand' example. Place both palm facing up, and hands next to each other. Now flip either side over to the other. One hand should be showing the back of the hand, while the other one is showing the palm.

They are not same and non-superimposable. This is where the concept of chirality comes in as one of the most essential and defining idea of stereoisomerism. Chirality essentially means 'mirror-image, non-superimposable molecules', and to say that a molecule is chiral is to say that its mirror image it must have one is not the same as it self. Whether a molecule is chiral or achiral depends upon a certain set of overlapping conditions.

Figure 4 shows an example of two molecules, chiral and achiral, respectively. Notice the distinct characteristic of the achiral molecule: it possesses two atoms of same element. In theory and reality, if one were to create a plane that runs through the other two atoms, they will be able to create what is known as bisecting plane: The images on either side of the plan is the same as the other Figure 4.

In this case, the molecule is considered 'achiral'. In other words, to distinguish chiral molecule from an achiral molecule, one must search for the existence of the bisecting plane in a molecule. All chiral molecules are deprive of bisecting plane, whether simple or complex.

As a universal rule, no molecule with different surrounding atoms are achiral. Chirality is a simple but essential idea to support the concept of stereoisomerism, being used to explain one type of its kind. The chemical properties of the chiral molecule differs from its mirror image, and in this lies the significance of chilarity in relation to modern organic chemistry.

We turn our attention next to molecules which have more than one stereocenter. We will start with a common four-carbon sugar called D-erythrose. A note on sugar nomenclature: biochemists use a special system to refer to the stereochemistry of sugar molecules, employing names of historical origin in addition to the designators ' D ' and ' L '.

You will learn about this system if you take a biochemistry class. As you can see, D -erythrose is a chiral molecule: C 2 and C 3 are stereocenters, both of which have the R configuration. In addition, you should make a model to convince yourself that it is impossible to find a plane of symmetry through the molecule, regardless of the conformation. Does D-erythrose have an enantiomer? Of course it does — if it is a chiral molecule, it must.

The enantiomer of erythrose is its mirror image, and is named L-erythrose once again, you should use models to convince yourself that these mirror images of erythrose are not superimposable. Notice that both chiral centers in L-erythrose both have the S configuration. In a pair of enantiomers, all of the chiral centers are of the opposite configuration. What happens if we draw a stereoisomer of erythrose in which the configuration is S at C 2 and R at C 3?

This stereoisomer, which is a sugar called D-threose, is not a mirror image of erythrose. D-threose is a diastereomer of both D-erythrose and L-erythrose. Upon comparison, bromochlorofluoromethane lacks a plane of symmetry while dichlorofluoromethane has a plane of symmetry. Identifying chiral carbons in a molecule is an important skill for organic chemists. The presence of a chiral carbon presents the possibility of a molecule having multiple stereoisomers. Most of the chiral centers we shall discuss in this chapter are asymmetric carbon atoms, but it should be recognized that other tetrahedral or pyramidal atoms may become chiral centers if appropriately substituted.

Also, when more than one chiral center is present in a molecular structure, care must be taken to analyze their relationship before concluding that a specific molecular configuration is chiral or achiral. This aspect of stereoisomerism will be treated later. Because an carbon requires four different substituents to become asymmertric, it can be said, with few exceptions, that sp 2 and sp hybridized carbons involved in multiple bonds are achiral.

Also, any carbon with more than one hydrogen, such as a -CH 3 or -CH 2 - group, are also achiral. Looking for planes of symmetry in a molecule is useful, but often difficult in practice.

It is difficult to illustrate on the two dimensional page, but you will see if you build models of these achiral molecules that, in each case, there is at least one plane of symmetry , where one side of the plane is the mirror image of the other.

In most cases, the easiest way to decide whether a molecule is chiral or achiral is to look for one or more stereocenters - with a few rare exceptions, the general rule is that molecules with at least one stereocenter are chiral, and molecules with no stereocenters are achiral. Determining if a carbon is bonded to four distinctly different substituents can often be difficult to ascertain.

Remember even the slightest difference makes a substituent unique. Often these difference can be distant from the chiral carbon itself. Careful consideration and often the building of molecular models may be required. A good example is shown below. It may appear that the molecule is achiral, however, when looking at the groups directly attached to the possible chiral carbon, it is clear that they all different.

The two alkyl groups are differ by a single -CH 2 - group which is enough to consider them different. When determinig the chirality of a molecule, it best to start by locating any chiral carbons. An obvious candidate is the ring carbon attached to the methyl substituent. The question then becomes: does the ring as two different substituents making the substituted ring carbon chiral? With an uncertantity such as this, it is then helpful try to identify any planes of symmetry in the molecule.

This molecule does have a plane of symmetry making the molecul achiral. The plane of symmetry would be easier see if the molecule were view from above. Typically, monosubstitued cycloalkanes have a similar plane of symmetry making them all achiral. Determine if each of the following molecules are chiral or achiral. For chiral molecules indicate any chiral carbons. Structures F and G are achiral.

The former has a plane of symmetry passing through the chlorine atom and bisecting the opposite carbon-carbon bond. The similar structure of compound E does not have such a symmetry plane, and the carbon bonded to the chlorine is a chiral center the two ring segments connecting this carbon are not identical. Structure G is essentially flat. All the carbons except that of the methyl group are sp 2 hybridized, and therefore trigonal-planar in configuration.

Thalidomide had previously been used in other countries as an antidepressant, and was believed to be safe and effective for both purposes. The drug was not approved for use in the U. It was not long, however, before doctors realized that something had gone horribly wrong: many babies born to women who had taken thalidomide during pregnancy suffered from severe birth defects.

Researchers later realized the problem lay in the fact that thalidomide was being provided as a mixture of two different isomeric forms. One of the isomers is an effective medication, the other caused the side effects.

Both isomeric forms have the same molecular formula and the same atom-to-atom connectivity, so they are not constitutional isomers. Where they differ is in the arrangement in three-dimensional space about one tetrahedral, sp 3 -hybridized carbon. These two forms of thalidomide are stereoisomers. If you make models of the two stereoisomers of thalidomide, you will see that they too are mirror images, and cannot be superimposed. As a historical note, thalidomide was never approved for use in the United States.

This was thanks in large part to the efforts of Dr. Frances Kelsey, a Food and Drug officer who, at peril to her career, blocked its approval due to her concerns about the lack of adequate safety studies, particularly with regard to the drug's ability to enter the bloodstream of a developing fetus. Unfortunately, though, at that time clinical trials for new drugs involved widespread and unregulated distribution to doctors and their patients across the country, so families in the U.

Very recently a close derivative of thalidomide has become legal to prescribe again in the United States, with strict safety measures enforced, for the treatment of a form of blood cancer called multiple myeloma. In Brazil, thalidomide is used in the treatment of leprosy - but despite safety measures, children are still being born with thalidomide-related defects.

Label the molecules below as chiral or achiral, and locate all stereocenters. Here are some more examples of chiral molecules that exist as pairs of enantiomers. I could draw it like this: H and H. So they're bonded to two of the same group, so none of these CH2's are good candidates for being a chiral center or chiral carbon.

They're both bonded to-- or all of them are bonded to two hydrogens and two other very similar-looking CH2 groups, although you have to look at the entire group that it's bonded to. But they're all definitely bonded to two hydrogens, so it's not four different groups. If we look at this CH right here, we could separate it out like this.

We could separate the H out like this, and so since it's bonded to a hydrogen. This carbon is bonded to a chlorine, and then it's bonded to-- well, it's not clear when you look at it right from the get-go whether this group is different than this group.

But if you go around, if you were to split it half-way like this, or maybe another better way to think about is if you were to go around this molecule in that direction, the counterclockwise direction, you would encounter a CH2 group, and then you encounter a CH2 group, and then you would encounter a third, and then you would encounter a fourth CH2 group, then you would come back to where you were before.

So you would encounter four CH2's and then you'd come back to where you were before. If you go in this direction, what happens? You encounter one, two, three, four CH2's and you come back to where you were before. So all of this, this bottom group, depending on how far you want to extend it, and this top group, are really the same group. So this is not a chiral center or not a chiral carbon. It's not bonded to four different groups. And this is also not a chiral molecule, because it does not have a chiral center.

And to see that it's not a chiral molecule-- let me see if I can backtrack this back to the way I wrote it right before. So you see that it's not a chiral molecule. There's a couple of ways you could think about it. The easiest way, or the way my brain likes to think about it, is just to think about its mirror image. Its mirror image will look like this. So if that's the mirror, you would have a chlorine. Now, in this situation, is there any way to rotate this to get this over there? Well, if you just took this molecule right here and you just rotated it degrees, what would it look like?

Well, maybe a little over-- yeah, well, not quite degrees, but if you were to rotate it so that the chlorine goes about that far, you would get this exact molecule. You would get something. It would look a little bit different. It would look like this. Let me see if I can do it justice. You would have a CH2. So let me let me do it up here where I have a little bit more space.

If I were to rotate this about that far, I would get a CH. If you were to rotate this all the way around, or actually this is almost exactly degrees, it would look like this. And the only difference between this and this is just how we drew this bond here. I could have easily, instead of drawing that bond like that, I could draw it facing up like that, and these are the exact same molecule.

So this molecule is also not chiral. So let's go to this one over here. So what is this? This is a bromochlorofluoromethane, just to practice our naming a little bit. But it's very clear that we are bonded to four different groups. All of the different groups, or the atoms in this case that are bonded to this carbon, are different, so this carbon is a chiral center. And it should also be pretty clear that it is also a chiral molecule. If you were to take its mirror image, and this is very similar to the example we did in the first video on chirality, but its mirror image will look like this.

You have the bromine on the right now.